Paraben is both an industrially produced and naturally occurring ester. Used as a preservative, it's usually near the bottom of the ingredient list in shampoos, soaps, toothpaste and deodorant. Allergic reactions to it are relatively rare when you consider how common it is, but different types of parabens also often appear in the same product, increasing the chance of a reaction.
One thing that should be mentioned regarding parabens is that a 2004 study in the United Kingdom linked them to breast cancer after trace amounts of methylparaben were found in breast cancer tumor biopsies [source: Yarosh]. Although further research has produced no conclusive evidence that parabens cause cancer, many consumers are still worried, preferring to take a better-safe-than-sorry approach to skin care. Whether or not the claims about the dangers of parabens are true, cosmetics companies have compensated for the backlash and now offer a wide variety of paraben-free products.
Parabens go by a long list of chemical pseudonyms, so if you think you're allergic to paraben, check your soaps and medicine cabinet for anything with paraben or parahydroxybenzoic in it.